1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which carries out feedback control of the air-fuel ratio of a mixture supplied to the engine, based on an adaptive control theory.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines is known, e.g. from U.S. Pat. No. 5,636,621, which calculates an air-fuel ratio control amount by using an adaptive controller including an adaptive parameter-adjusting mechanism of a recurrence formula type based on an adaptive control theory, and controls the air-fuel ratio of a mixture supplied to the engine to a desired air-fuel ratio in a feedback manner. In the known air-fuel ratio control system, an air-fuel ratio sensor arranged in the exhaust system of the engine detects the air-fuel ratio and supplies an output indicative of the detected air-fuel ratio to the adaptive controller, which carries out the air-fuel ratio feedback control in response to the detected air-fuel ratio.
In the known air-fuel ratio control system, a determination as to the stableness of adaptive control is carried out based on adaptive parameters calculated by the adaptive parameter-adjusting mechanism, and if it is determined that the adaptive control is unstable, the adaptive parameters are initialized, i.e. reset to initial values thereof.
Further, U.S. Pat. No. 5,636,621, discloses a method which, at the start of the adaptive control, initializes the adaptive parameters according to the desired air-fuel ratio such that the air-fuel ratio control amount which is output from the adaptive controller assumes a central value thereof.
According to the initialization disclosed in U.S. Pat. No. 5,636,621, however, the initial values of the adaptive parameters are fixed values, and therefore, after the initialization, it takes a considerable time period for the adaptive parameters to converge to respective optimum values, whereby the engine can temporarily undergo degraded controllability.
Further, according to the initialization disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 8-291747, the air-fuel ratio control amount is set to the central value thereof. As a result, when the air-fuel ratio control is switched, e.g. from ordinary PID control to the adaptive control, it can occur that the air-fuel ratio control amount is suddenly changed at the switching point of time (at which the adaptive control starts).